NAT (Network Address Translation) devices have been known each of which performs translation between a network address used on one network and a network address used on another network, at a connection point connecting different networks. In addition, multi-homed networks have been known each of which is connected to another network through a plurality of connection points.
In addition, a communication method has been known where when a terminal movable among different networks performs packet transmission between any different networks, a transmission source address of a packet transmitted by the terminal is translated into a same global address. In a first network, a first address change mechanism is provided that changes the transmission source address of a packet transmitted from the terminal, to the global address of a packet. In addition, a second address change mechanism is provided in a second network, and when the terminal has moved from the first network to the second network, the second address change mechanism changes the transmission source address of a packet transmitted from the terminal, to the same address as the global address. The second address change mechanism detects that a registration request has been transmitted from the terminal to a mobile IP agent of the second network, and after the detection, the second address change mechanism acquires the global address from the first address change mechanism. Alternatively, the second address change mechanism detects that a registration response has been transmitted from a mobile IP agent of the first network to the mobile IP agent of the second network, and after the detection, the second address change mechanism acquires the global address from the first address change mechanism.
An address translation method has been known where in a home network, a home agent and an address translation function unit are provided and in a foreign network of a movement destination, a foreign agent and an address translation function unit are provided. The foreign network inquires of the agent of the home network about the global address of a mobile terminal that has moved to the foreign network, and stores the global address of the mobile terminal in the address translation function unit. Examples of such methods include techniques disclosed in Japanese Laid-open Patent Publication No. 2001-274834 and Japanese Laid-open Patent Publication No. 2002-94546.